In recent years, a variety methods for examination of bio-related materials such as DNA, sugar chains, and proteins have been developed for certain purposes such as diagnosis of diseases, detection of individual differences in drug metabolism, or food or environmental monitoring, and in particular, biosensors for detecting biomolecules based on electric signals have been studied. Recently, many studies have been made on biosensors for detecting biological reactions by means of a field effect transistor (hereinafter also referred to as “FET”), in view of fast conversion of electric signals and easiness in connection between an integrated circuit and a micro electro mechanical system (MEMS).
A conventional FET biosensor has a structure comprising: a MOSFET from which the gate electrode is removed; and an ion sensitive membrane deposited on the insulating film, and such a structure is called “ISFET (Ion Sensitive FET).” Such a structure can function as various biosensors when oxidoreductases, various proteins, DNAs, antigens, antibodies, etc. are placed on the ion sensitive membrane (for example, Patent Literature 1).
Specifically, a FET used in a biosensor comprises a silicon substrate; a source electrode, a drain electrode, and a gate insulating film, which are formed on the surface of the silicon substrate; and a metal electrode formed on the surface of the gate insulator between the source and drain electrodes. A DNA probe and alkanethiol are placed on the surface of the metal electrode. In practical measurement, the metal electrode, the DNA probe and alkanethiol placed on the surface of the metal electrode, and a reference electrode are placed in a reaction solution in a measurement cell. When a high frequency voltage is applied through the reference electrode, the electrical characteristics of the insulated gate field-effect transistor will change before and after the target DNA contained in the reaction solution binds to the DNA probe. Therefore, whether or not the target DNA in the reaction solution is extended can be determined by detecting a change in the electrical characteristics of the transistor, specifically a change in the value of the current through the source and the drain.